The distribution of radiant energy in plant canopies determines one of the fundamental interactions of biophysical ecology—that of energy exchange between photosynthetic organisms and their environment. Accurate knowledge of light absorption by plant canopies permits the calculation of important plant- and ecosystem-level properties, including rates of primary production, which will be the focus of this chapter. Knowledge of the interaction between light and plant canopies is also crucial for remote sensing, quantification of vegetation abundance and distribution, as well as for the development of inversion, techniques to infer plant chemical composition, important for ecosystem-scale estimates of plant growth and biogeochemical fluxes (Jacquemoudetal., 1996; Lacapraetal., 1996; Broge and Leblanc, 2000). Submerged aquatic vegetation, including seagrass beds, provide a strong optical signature that can be tracked using satellite and airborne remote sensing (Armstrong, 1993; Mumby et al., 1997; Chauvaud et al., 2001; Dierssen et al., 2003), and this will be the subject of Dekker et al., Chapter 15.
Seagrasses represent an ecologically important structuring element and major source of primary production in shallow waters, worldwide. The primacy of light availability in determining seagrass bed density, distribution and productivity is particularly acute (Hemminga and Duarte, 2000). Although minimum light requirements for most marine macro-phytes are on the order of 0.1 to 1% of in-water surface irradiance [£d(0)], seagrasses have unusually high light requirements, ranging from 10% to as much as 37% of £d(0) (Duarte, 1991b; Olesen
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and Sand-Jensen, 1993; Kenworthy and Fonseca, 1996). These high light requirements, which can be traced, at least partially, to inefficient carbon-concentrating mechanisms for photosynthesis (Du-rako, 1993; Beer and Rehnberg, 1997; Zimmerman et al., 1997; Invers et al., 2001; Larkum et al., Chapter 14), make seagrasses particularly vulnerable to deteriorated water quality and light competition from micro- and macroalgal blooms induced by eutrophication (Short and Wyllie-Echeverria, 1996; Ralph et al., Chapter 24). Consequently, the development of robust mechanistic relationships between the submarine light field and photosynthesis of submerged plant canopies will facilitate our fundamental understanding of coastal biogeochemical processes and assist in the management of these important coastal resources.
Light-dependent productivity of seagrass beds has been estimated from photosynthesis vs. irra-diance (P vs. E) relationships measured at scales ranging from individual leaves (e.g. Dennison and Alberte, 1982, 1985; Zimmerman et al., 1994; Zimmerman et al., 2001) to individual multi-leaved shoots (Fourqurean and Zieman, 1991) to in situ ben-thic chambers enclosing multiple shoots (Dunton, 1994; Herzka and Dunton, 1997; Mateo et al., Chapter 7). Each approach can provide reasonable local estimates of whole plant photosynthesis, carbon balance and light requirements, which has made this general approach extremely useful for exploring the relationship between environmental forcing and primary productivity in tightly focused local studies. Unfortunately, the functional relationships behind these relatively simple "big leaf" budgets are not readily transported to seagrass beds growing in different light environments because they do not account for the interactions between the overlying water column and the distribution and orientation of the
A. WD. Larkum et al. (eds.), Seagrasses: Biology, Ecology and Conservation, pp. 303-321. © 2006 Springer. Printed in the Netherlands.
plant canopy relative to the incident light field, or to the spectral quality of the incident light. Further, it is difficult to evaluate potentially important density dependent effects (e.g. self-shading) with these "big leaf" models. Finally, the data required to develop and validate the empirical models are time consuming to collect and not easily automated, which limits their utility for resource management objectives.
We can develop a more mechanistic understanding of seagrass bed productivity by employing some biophysical principles and geometric reasoning to characterize the general interaction of seagrass canopies with the submarine light field and water column in which they are embedded. This chapter focuses on the vertical distribution and orientation of leafbiomass (Fig. 3), and the optical properties of leaves that determine spectral light absorption by the plant canopy (Fig. 4) within an optically active water column. An overview of general terms and principles of radiative transfer theory relevant to this discussion is presented in Zimmerman and Dekker, Chapter 12. More detailed treatments of radiative transfer theory applied to natural waters can be found in Kirk (1994) and Mobley (1994).
6 = 1. Conversely, the absorptance (A), or probability of a photon being absorbed by the canopy is 1 minus the probability of transmission:
The leaves of real plant canopies, however, are almost never horizontally oriented, nor randomly distributed, especially in the vertical. And their optical density is neither black nor even spectrally neutral. Additionally, the angular distribution of natural sunlight is composed of both direct and diffuse components, which complicates the estimation of 6. Furthermore, leaves scatter a significant portion of the incident beam in both the forward and backward directions, changing the angular distribution of light as it passes through the canopy. Finally, the natural water column in which seagrass canopies are suspended is also a source of light attenuation and scattering. The development of accurate relationships describing the interaction between submerged plant canopies and the incident light field requires that we account for these complications.
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